In conventional x-ray therapy, rectangular field shapes are formed by four motor driven jaws in the radiation head. Irregular field shapes for individual portals are then produced by mounting shadow blocks on a shadow tray between the jaws and the patient. The shadow blocks shield critical organs not invaded by the tumor. The radiation beam can be directed at the prescribed treatment volume from a single direction (single port therapy), from two or more directions (multi-port therapy), or the beam can be swept through an arc (arc or rotation therapy), all by rotating an isocentric gantry, for example. A cylindrical-shaped region of high dose is produced by a rectangular field in multi-port, arc or rotation therapy.
In multi-port therapy, the shadow blocks are changed for each beam angle. This can require that the technologist go back into the shielded treatment room for each treatment field, a time-consuming process. If the beam angle is not vertical, the shadow blocks must be locked to the shadow tray (to avoid their falling off), which can be awkward and time-consuming. The shadow blocks are typically made by pouring a heavy metal into a pre-cut mold, which is also time-consuming. The shadow blocks can be heavy, difficult to handle, and dangerous if they fall on the patient or the radiotherapy personnel.
In arc or rotation therapy, it is not practicable to change the shadow blocks continually or in small steps of beam angle.
The usual treatment field shapes result in a three-dimensional treatment volume which includes considerable volume of normal tissue, thereby limiting the dose that can be given to the tumor volume. The irradiation dose that can be delivered to a portion of an organ of normal tissue without serious damage can be increased if the size of that portion of the organ receiving such radiation dose can be reduced. Avoidance of serious damage to the organs surrounding and overlying the tumor determines the maximum dose that can be delivered to the tumor. Cure rates for many tumors are a steep function of the dose delivered to the tumor. Techniques are under development to make the treatment volume conform more closely to the shape of the tumor volume, thereby minimizing the product of volume and dose to normal tissue, with its attendant effects on the health of the patient. This can permit higher dose to tumors or can result in less damage to normal tissue. These techniques involve moving the x-ray jaws during treatment or scanning the x-ray beam or using multi-leaf jaws.
In a technique called dynamic therapy, one set of jaws is set to form a narrow (e.g., 4 cm) fan beam and the spread of the fan beam is varied by the second set of jaws to conform to the boundaries of the prescribed treatment volume as the beam angle is swept or stepped around the patient and as the patient and associated table top are moved through the fan beam. A computer controls the movements of the table top in x, y and z, the gantry angle, the upper jaws during start and stop of the scan, the lower jaws throughout the scan, and the dose rate. The complexity is such that great care must be exercised in preparing for such treatments, which consumes considerable time.
A technique has been proposed in which a narrow collimated lobe of x-rays is scanned over the treatment field, permitting production of irregular field shapes at selected beam angles. Because only a small fraction of the x-ray output is within the narrow lobe, the effective dose rate is low and the time to produce a portal field is hence long and multi-port treatment times are excessively long. Also, scanning individual fields is not readily applicable to arc and rotation therapy modes.
Machines have been built in which each of the lower pair of jaws is divided into a number (e.g., 5 to 32) of narrow bars called leaves. Each leaf may be about 10 cm thick (in the beam direction) to provide adequate attenuation of the x-ray beam (down to about 1%), about 0.5 to 1.5 cm wide and about 14 cm long. Each leaf can be moved independently by a motor drive. This permits the production of irregularly shaped fields with stepped boundaries, thereby avoiding shadow blocks for many situations in portal therapy. The shape can be changed as the beam direction is swept in arc or rotation therapy. The disadvantage of this technique of replacing the lower jaws by a multiplicity of leaves is that each leaf is quite large and heavy, requiring a motor drive system which consumes considerable space. There is limited room in the radiation head for all these components so either sacrifices in performance are made (such a fewer leaves, limited field size) or the construction costs become large.
In a different technique, the conventional upper and lower pairs of jaws are retained and a set of leaves is mounted between the jaws and the patient. Each leaf moves in a plane, driven by a rotating cam or pushed by a form corresponding to the desired irregular field shape. In one early concept, each leaf was thick enough to attenuate the x-ray beam to the required level (to about 5% of unattenuated beam intensity), the ends and sides of the leaf forming a rectangular parallelpiped, hence not aimed toward the x-ray source. In a recent concept, a multiplicity of small diameter rods forms a stack sufficiently thick to provide the required beam attenuation. Each rod can slide with respect to its neighbors. A form corresponding to the desired field shape boundary is used to push the assembly of rods so that their ends form a similar beam boundary. Since the rods are small in diameter, the radiation field boundary can be relatively smooth (very small steps) and tapered (focused) toward the source. However, varying the field shape as a function of beam angle without entering the treatment room can require a quite complex drive system because the large number of rods requires that they be driven enmasse instead of individually.